Cyclic cytidine 3 ', 5' monophosphate (cyclic CMP) has been implicated in cell growth. Cytidylate cyclase, the enzyme responsible for the synthesis of cyclic CMP was not detected in both fetal and regenerating liver even with the use of a very highly sensitive method for the assay of the enzyme. However, the enzyme was detected in the leukemia cells (L1210) grown either in tissue culture or in the intraperitonial cavity of mice. The level of enzyme activity was however very small. Since it was not possible to raise antibody against cyclic CMP, it has not been possible to assay the presence of cyclic CMP in liver or leukemia cell culture. It is possible that cyclic CMP may be involved in malignant cell growth but more experiments are necessary to implicate cell growth with increase in cylic CMP.